The present invention relates generally to the determination of the temperature of railroad car wheel journals and more particularly to that class of devices generally known as "hotbox" detectors.
It is well known in the art to place detectors adjacent railroad tracks to identify overheated wheel journals. These detector devices generally include infrared sensitive elements which are focused to scan the journal box. The detector includes suitable circuitry to develop a signal which is representative of the journal temperature and this signal may be used in a variety of ways such as sounding an alarm in the event a signal indicates the temperature of a journal exceeds a predetermined value or to provide a visual recording of the temperature of each of the journals as the train passes the wayside station where the detector is located.
The most common form of hotbox detector in use today is the so-called bolometer type which will be described in some detail hereinafter. Briefly, however, the bolometer type hotbox detector uses temperature sensitive resistors (thermistors) in a bridge arrangement. While the performance of a bolometer type hotbox detector is, essentially, satisfactory, it is a relatively expensive apparatus. The thermistor is a very high resistance device and, therefore, requires a high voltage supply. Because of the bridge arrangement, the supplies must be very accurate and must not drift from their designated voltage. Because the signal-to-noise ratio of the bolometer decreases as the temperature decreases (at certain temperatures within the normal operating range of hotbox detector devices the ratio becomes unsatisfactory), heaters are required to maintain the ambient temperature of the thermistors above a predetermined minimum. In addition to the expense associated with the heaters, the use of heaters may require more elaborate optics as will be explained later. In addition, the frequency sensitivity of a bolometer type of system is not as broad as is desirable.
Other forms of infrared responsive devices are, of course, known. An example of one of these is the pyroelectric cell which outputs a charge in response to incident radiation in the infrared range. While pyroelectric cells overcome many if not all of the disadvantages of the bolometer system, they present certain problems of their own. Such cells are, generally, sensitive to shock and generate noise signals in response thereto. This, obviously, can be a problem when the device is located adjacent a railroad track. There are, however, materials which do not exhibit large shock-noise problems and with the use of suitable shock absorbing mountings this problem can be reduced to acceptable levels. A more serious problem is the fact that commercially available pyroelectric cells have an extremely poor gain response when considered over any sizable range of signal input frequencies. A typical infrared sensor employing a pyroelectric device has a flat or constant gain response only over about two percent of the frequency range required for good hotbox detector operation (e.g., 0.5 to 300 Hz). Thus, the pyroelectric cell without suitable compensating circuitry is an unsatisfactory substitute for the existing bolometer systems.